Internet-Draft DetNet Use Cases September 2017
Abstract
This draft documents requirements in several diverse industries to
establish multi-hop paths for characterized flows with deterministic
properties. In this context deterministic implies that streams can
be established which provide guaranteed bandwidth and latency which
can be established from either a Layer 2 or Layer 3 (IP) interface,
and which can co-exist on an IP network with best-effort traffic.
Additional requirements include optional redundant paths, very high
reliability paths, time synchronization, and clock distribution.
Industries considered include professional audio, electrical
utilities, building automation systems, wireless for industrial,
cellular radio, industrial machine-to-machine, mining, private
blockchain, and network slicing.
For each case, this document will identify the application, identify
representative solutions used today, and the improvements IETF DetNet
solutions may enable.
Status of This Memo
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This Internet-Draft will expire on March 22, 2018.
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The level of detail in each use case should be sufficient to express
the relevant elements of the use case, but not more.
At the end we consider the use cases collectively, and examine the
most significant goals they have in common.
2. Pro Audio and Video2.1. Use Case Description
The professional audio and video industry ("ProAV") includes:
o Music and film content creation
o Broadcast
o Cinema
o Live sound
o Public address, media and emergency systems at large venues
(airports, stadiums, churches, theme parks).
These industries have already transitioned audio and video signals
from analog to digital. However, the digital interconnect systems
remain primarily point-to-point with a single (or small number of)
signals per link, interconnected with purpose-built hardware.
These industries are now transitioning to packet-based infrastructure
to reduce cost, increase routing flexibility, and integrate with
existing IT infrastructure.
Today ProAV applications have no way to establish deterministic
streams from a standards-based Layer 3 (IP) interface, which is a
fundamental limitation to the use cases described here. Today
deterministic streams can be created within standards-based layer 2
LANs (e.g. using IEEE 802.1 AVB) however these are not routable via
IP and thus are not effective for distribution over wider areas (for
example broadcast events that span wide geographical areas).
It would be highly desirable if such streams could be routed over the
open Internet, however solutions with more limited scope (e.g.
enterprise networks) would still provide a substantial improvement.
The following sections describe specific ProAV use cases.
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Internet-Draft DetNet Use Cases September 20172.1.1. Uninterrupted Stream Playback
Transmitting audio and video streams for live playback is unlike
common file transfer because uninterrupted stream playback in the
presence of network errors cannot be achieved by re-trying the
transmission; by the time the missing or corrupt packet has been
identified it is too late to execute a re-try operation. Buffering
can be used to provide enough delay to allow time for one or more
retries, however this is not an effective solution in applications
where large delays (latencies) are not acceptable (as discussed
below).
Streams with guaranteed bandwidth can eliminate congestion on the
network as a cause of transmission errors that would lead to playback
interruption. Use of redundant paths can further mitigate
transmission errors to provide greater stream reliability.
2.1.2. Synchronized Stream Playback
Latency in this context is the time between when a signal is
initially sent over a stream and when it is received. A common
example in ProAV is time-synchronizing audio and video when they take
separate paths through the playback system. In this case the latency
of both the audio and video streams must be bounded and consistent if
the sound is to remain matched to the movement in the video. A
common tolerance for audio/video sync is one NTSC video frame (about
33ms) and to maintain the audience perception of correct lip sync the
latency needs to be consistent within some reasonable tolerance, for
example 10%.
A common architecture for synchronizing multiple streams that have
different paths through the network (and thus potentially different
latencies) is to enable measurement of the latency of each path, and
have the data sinks (for example speakers) delay (buffer) all packets
on all but the slowest path. Each packet of each stream is assigned
a presentation time which is based on the longest required delay.
This implies that all sinks must maintain a common time reference of
sufficient accuracy, which can be achieved by any of various
techniques.
This type of architecture is commonly implemented using a central
controller that determines path delays and arbitrates buffering
delays.
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Internet-Draft DetNet Use Cases September 20172.1.3. Sound Reinforcement
Consider the latency (delay) from when a person speaks into a
microphone to when their voice emerges from the speaker. If this
delay is longer than about 10-15 milliseconds it is noticeable and
can make a sound reinforcement system unusable (see slide 6 of
[SRP_LATENCY]). (If you have ever tried to speak in the presence of
a delayed echo of your voice you may know this experience).
Note that the 15ms latency bound includes all parts of the signal
path, not just the network, so the network latency must be
significantly less than 15ms.
In some cases local performers must perform in synchrony with a
remote broadcast. In such cases the latencies of the broadcast
stream and the local performer must be adjusted to match each other,
with a worst case of one video frame (33ms for NTSC video).
In cases where audio phase is a consideration, for example beam-
forming using multiple speakers, latency requirements can be in the
10 microsecond range (1 audio sample at 96kHz).
2.1.4. Deterministic Time to Establish Streaming
Note: The WG has decided that guidelines for deterministic time to
establish stream startup is not within scope of DetNet. If bounded
timing of establishing or re-establish streams is required in a given
use case, it is up to the application/system to achieve this. (The
supporting text from this section has been removed as of draft 12).
2.1.5. Secure Transmission2.1.5.1. Safety
Professional audio systems can include amplifiers that are capable of
generating hundreds or thousands of watts of audio power which if
used incorrectly can cause hearing damage to those in the vicinity.
Apart from the usual care required by the systems operators to
prevent such incidents, the network traffic that controls these
devices must be secured (as with any sensitive application traffic).
2.2. Pro Audio Today
Some proprietary systems have been created which enable deterministic
streams at Layer 3 however they are "engineered networks" which
require careful configuration to operate, often require that the
system be over-provisioned, and it is implied that all devices on the
network voluntarily play by the rules of that network. To enable
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these industries to successfully transition to an interoperable
multi-vendor packet-based infrastructure requires effective open
standards, and we believe that establishing relevant IETF standards
is a crucial factor.
2.3. Pro Audio Future2.3.1. Layer 3 Interconnecting Layer 2 Islands
It would be valuable to enable IP to connect multiple Layer 2 LANs.
As an example, ESPN recently constructed a state-of-the-art 194,000
sq ft, $125 million broadcast studio called DC2. The DC2 network is
capable of handling 46 Tbps of throughput with 60,000 simultaneous
signals. Inside the facility are 1,100 miles of fiber feeding four
audio control rooms (see [ESPN_DC2] ).
In designing DC2 they replaced as much point-to-point technology as
they could with packet-based technology. They constructed seven
individual studios using layer 2 LANS (using IEEE 802.1 AVB) that
were entirely effective at routing audio within the LANs. However to
interconnect these layer 2 LAN islands together they ended up using
dedicated paths in a custom SDN (Software Defined Networking) router
because there is no standards-based routing solution available.
2.3.2. High Reliability Stream Paths
On-air and other live media streams are often backed up with
redundant links that seamlessly act to deliver the content when the
primary link fails for any reason. In point-to-point systems this is
provided by an additional point-to-point link; the analogous
requirement in a packet-based system is to provide an alternate path
through the network such that no individual link can bring down the
system.
2.3.3. Integration of Reserved Streams into IT Networks
A commonly cited goal of moving to a packet based media
infrastructure is that costs can be reduced by using off the shelf,
commodity network hardware. In addition, economy of scale can be
realized by combining media infrastructure with IT infrastructure.
In keeping with these goals, stream reservation technology should be
compatible with existing protocols, and not compromise use of the
network for best effort (non-time-sensitive) traffic.
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Internet-Draft DetNet Use Cases September 20172.3.4. Use of Unused Reservations by Best-Effort Traffic
In cases where stream bandwidth is reserved but not currently used
(or is under-utilized) that bandwidth must be available to best-
effort (i.e. non-time-sensitive) traffic. For example a single
stream may be nailed up (reserved) for specific media content that
needs to be presented at different times of the day, ensuring timely
delivery of that content, yet in between those times the full
bandwidth of the network can be utilized for best-effort tasks such
as file transfers.
This also addresses a concern of IT network administrators that are
considering adding reserved bandwidth traffic to their networks that
("users will reserve large quantities of bandwidth and then never un-
reserve it even though they are not using it, and soon the network
will have no bandwidth left").
2.3.5. Traffic Segregation
Note: It is still under WG discussion whether this topic will be
addressed by DetNet.
Sink devices may be low cost devices with limited processing power.
In order to not overwhelm the CPUs in these devices it is important
to limit the amount of traffic that these devices must process.
As an example, consider the use of individual seat speakers in a
cinema. These speakers are typically required to be cost reduced
since the quantities in a single theater can reach hundreds of seats.
Discovery protocols alone in a one thousand seat theater can generate
enough broadcast traffic to overwhelm a low powered CPU. Thus an
installation like this will benefit greatly from some type of traffic
segregation that can define groups of seats to reduce traffic within
each group. All seats in the theater must still be able to
communicate with a central controller.
There are many techniques that can be used to support this
requirement including (but not limited to) the following examples.
2.3.5.1. Packet Forwarding Rules, VLANs and Subnets
Packet forwarding rules can be used to eliminate some extraneous
streaming traffic from reaching potentially low powered sink devices,
however there may be other types of broadcast traffic that should be
eliminated using other means for example VLANs or IP subnets.
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Internet-Draft DetNet Use Cases September 20172.3.5.2. Multicast Addressing (IPv4 and IPv6)
Multicast addressing is commonly used to keep bandwidth utilization
of shared links to a minimum.
Because of the MAC Address forwarding nature of Layer 2 bridges it is
important that a multicast MAC address is only associated with one
stream. This will prevent reservations from forwarding packets from
one stream down a path that has no interested sinks simply because
there is another stream on that same path that shares the same
multicast MAC address.
Since each multicast MAC Address can represent 32 different IPv4
multicast addresses there must be a process put in place to make sure
this does not occur. Requiring use of IPv6 address can achieve this,
however due to their continued prevalence, solutions that are
effective for IPv4 installations are also required.
2.3.6. Latency Optimization by a Central Controller
A central network controller might also perform optimizations based
on the individual path delays, for example sinks that are closer to
the source can inform the controller that they can accept greater
latency since they will be buffering packets to match presentation
times of farther away sinks. The controller might then move a stream
reservation on a short path to a longer path in order to free up
bandwidth for other critical streams on that short path. See slides
3-5 of [SRP_LATENCY].
Additional optimization can be achieved in cases where sinks have
differing latency requirements, for example in a live outdoor concert
the speaker sinks have stricter latency requirements than the
recording hardware sinks. See slide 7 of [SRP_LATENCY].
2.3.7. Reduced Device Cost Due To Reduced Buffer Memory
Device cost can be reduced in a system with guaranteed reservations
with a small bounded latency due to the reduced requirements for
buffering (i.e. memory) on sink devices. For example, a theme park
might broadcast a live event across the globe via a layer 3 protocol;
in such cases the size of the buffers required is proportional to the
latency bounds and jitter caused by delivery, which depends on the
worst case segment of the end-to-end network path. For example on
todays open internet the latency is typically unacceptable for audio
and video streaming without many seconds of buffering. In such
scenarios a single gateway device at the local network that receives
the feed from the remote site would provide the expensive buffering
required to mask the latency and jitter issues associated with long
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distance delivery. Sink devices in the local location would have no
additional buffering requirements, and thus no additional costs,
beyond those required for delivery of local content. The sink device
would be receiving the identical packets as those sent by the source
and would be unaware that there were any latency or jitter issues
along the path.
2.4. Pro Audio Asks
o Layer 3 routing on top of AVB (and/or other high QoS networks)
o Content delivery with bounded, lowest possible latency
o IntServ and DiffServ integration with AVB (where practical)
o Single network for A/V and IT traffic
o Standards-based, interoperable, multi-vendor
o IT department friendly
o Enterprise-wide networks (e.g. size of San Francisco but not the
whole Internet (yet...))
3. Electrical Utilities3.1. Use Case Description
Many systems that an electrical utility deploys today rely on high
availability and deterministic behavior of the underlying networks.
Here we present use cases in Transmission, Generation and
Distribution, including key timing and reliability metrics. We also
discuss security issues and industry trends which affect the
architecture of next generation utility networks
3.1.1. Transmission Use Cases3.1.1.1. Protection
Protection means not only the protection of human operators but also
the protection of the electrical equipment and the preservation of
the stability and frequency of the grid. If a fault occurs in the
transmission or distribution of electricity then severe damage can
occur to human operators, electrical equipment and the grid itself,
leading to blackouts.
Communication links in conjunction with protection relays are used to
selectively isolate faults on high voltage lines, transformers,
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reactors and other important electrical equipment. The role of the
teleprotection system is to selectively disconnect a faulty part by
transferring command signals within the shortest possible time.
3.1.1.1.1. Key Criteria
The key criteria for measuring teleprotection performance are command
transmission time, dependability and security. These criteria are
defined by the IEC standard 60834 as follows:
o Transmission time (Speed): The time between the moment where state
changes at the transmitter input and the moment of the
corresponding change at the receiver output, including propagation
delay. Overall operating time for a teleprotection system
includes the time for initiating the command at the transmitting
end, the propagation delay over the network (including equipments)
and the selection and decision time at the receiving end,
including any additional delay due to a noisy environment.
o Dependability: The ability to issue and receive valid commands in
the presence of interference and/or noise, by minimizing the
probability of missing command (PMC). Dependability targets are
typically set for a specific bit error rate (BER) level.
o Security: The ability to prevent false tripping due to a noisy
environment, by minimizing the probability of unwanted commands
(PUC). Security targets are also set for a specific bit error
rate (BER) level.
Additional elements of the the teleprotection system that impact its
performance include:
o Network bandwidth
o Failure recovery capacity (aka resiliency)
3.1.1.1.2. Fault Detection and Clearance Timing
Most power line equipment can tolerate short circuits or faults for
up to approximately five power cycles before sustaining irreversible
damage or affecting other segments in the network. This translates
to total fault clearance time of 100ms. As a safety precaution,
however, actual operation time of protection systems is limited to
70- 80 percent of this period, including fault recognition time,
command transmission time and line breaker switching time.
Some system components, such as large electromechanical switches,
require particularly long time to operate and take up the majority of
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the total clearance time, leaving only a 10ms window for the
telecommunications part of the protection scheme, independent of the
distance to travel. Given the sensitivity of the issue, new networks
impose requirements that are even more stringent: IEC standard 61850
limits the transfer time for protection messages to 1/4 - 1/2 cycle
or 4 - 8ms (for 60Hz lines) for the most critical messages.
3.1.1.1.3. Symmetric Channel Delay
Note: It is currently under WG discussion whether symmetric path
delays are to be guaranteed by DetNet.
Teleprotection channels which are differential must be synchronous,
which means that any delays on the transmit and receive paths must
match each other. Teleprotection systems ideally support zero
asymmetric delay; typical legacy relays can tolerate delay
discrepancies of up to 750us.
Some tools available for lowering delay variation below this
threshold are:
o For legacy systems using Time Division Multiplexing (TDM), jitter
buffers at the multiplexers on each end of the line can be used to
offset delay variation by queuing sent and received packets. The
length of the queues must balance the need to regulate the rate of
transmission with the need to limit overall delay, as larger
buffers result in increased latency.
o For jitter-prone IP packet networks, traffic management tools can
ensure that the teleprotection signals receive the highest
transmission priority to minimize jitter.
o Standard packet-based synchronization technologies, such as
1588-2008 Precision Time Protocol (PTP) and Synchronous Ethernet
(Sync-E), can help keep networks stable by maintaining a highly
accurate clock source on the various network devices.
3.1.1.1.4. Teleprotection Network Requirements (IEC 61850)
The following table captures the main network metrics as based on the
IEC 61850 standard.
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Internet-Draft DetNet Use Cases September 20173.1.1.1.6. Current Differential Protection Scheme
Current differential protection is commonly used for line protection,
and is typical for protecting parallel circuits. At both end of the
lines the current is measured by the differential relays, and both
relays will trip the circuit breaker if the current going into the
line does not equal the current going out of the line. This type of
protection scheme assumes some form of communications being present
between the relays at both end of the line, to allow both relays to
compare measured current values. Line differential protection
schemes assume a very low telecommunications delay between both
relays, often as low as 5ms. Moreover, as those systems are often
not time-synchronized, they also assume symmetric telecommunications
paths with constant delay, which allows comparing current measurement
values taken at the exact same time.
+----------------------------------+--------------------------------+
| Current Differential protection | Attribute |
| Requirement | |
+----------------------------------+--------------------------------+
| One way maximum delay | 5 ms |
| Asymetric delay Required | Yes |
| Maximum jitter | less than 250 us (750us for |
| | legacy IED) |
| Topology | Point to point, point to |
| | Multi-point |
| Bandwidth | 64 Kbps |
| Availability | 99.9999 |
| precise timing required | Yes |
| Recovery time on node failure | less than 50ms - hitless |
| performance management | Yes, Mandatory |
| Redundancy | Yes |
| Packet loss | 0.1% |
+----------------------------------+--------------------------------+
Table 3: Current Differential Protection metrics
3.1.1.1.7. Distance Protection Scheme
Distance (Impedance Relay) protection scheme is based on voltage and
current measurements. The network metrics are similar (but not
identical to) Current Differential protection.
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components are interconnected using fiber optics, industrial buses,
industrial Ethernet, EtherCat, or a combination of them. Industrial
signaling and control protocols such as Modbus, Profibus, Profinet
and EtherCat are used directly on top of the Layer 2 transport or
encapsulated over TCP/IP.
The Data collected from the sensors and condition monitoring systems
is multiplexed onto fiber cables for transmission to the base of the
tower, and to remote control centers. The turbine controller
continuously monitors the condition of the wind turbine and collects
statistics on its operation. This controller also manages a large
number of switches, hydraulic pumps, valves, and motors within the
wind turbine.
There is usually a controller both at the bottom of the tower and in
the nacelle. The communication between these two controllers usually
takes place using fiber optics instead of copper links. Sometimes, a
third controller is installed in the hub of the rotor and manages the
pitch of the blades. That unit usually communicates with the nacelle
unit using serial communications.
3.1.2.2.2. Inter-Domain network considerations
A remote control center belonging to a grid operator regulates the
power output, enables remote actuation, and monitors the health of
one or more wind parks in tandem. It connects to the local control
center in a wind park over the Internet (Figure 2) via firewalls at
both ends. The AS path between the local control center and the Wind
Park typically involves several ISPs at different tiers. For
example, a remote control center in Denmark can regulate a wind park
in Greece over the normal public AS path between the two locations.
The remote control center is part of the SCADA system, setting the
desired power output to the wind park and reading back the result
once the new power output level has been set. Traffic between the
remote control center and the wind park typically consists of
protocols like IEC 60870-5-104 [IEC-60870-5-104], OPC XML-DA
[OPCXML], Modbus [MODBUS], and SNMP [RFC3411]. Currently, traffic
flows between the wind farm and the remote control center are best
effort. QoS requirements are not strict, so no SLAs or service
provisioning mechanisms (e.g., VPN) are employed. In case of events
like equipment failure, tolerance for alarm delay is on the order of
minutes, due to redundant systems already in place.
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PROFIBUS/PROFINET, and other protocols are designed around a common
paradigm of request and respond. Each protocol is designed for a
master device such as an HMI (Human Machine Interface) system to send
commands to subordinate slave devices to retrieve data (reading
inputs) or control (writing to outputs). Because many of these
protocols lack authentication, encryption, or other basic security
measures, they are prone to network-based attacks, allowing a
malicious actor or attacker to utilize the request-and-respond system
as a mechanism for command-and-control like functionality. Specific
security concerns common to most industrial control, including
utility telecommunication protocols include the following:
o Network or transport errors (e.g. malformed packets or excessive
latency) can cause protocol failure.
o Protocol commands may be available that are capable of forcing
slave devices into inoperable states, including powering-off
devices, forcing them into a listen-only state, disabling
alarming.
o Protocol commands may be available that are capable of restarting
communications and otherwise interrupting processes.
o Protocol commands may be available that are capable of clearing,
erasing, or resetting diagnostic information such as counters and
diagnostic registers.
o Protocol commands may be available that are capable of requesting
sensitive information about the controllers, their configurations,
or other need-to-know information.
o Most protocols are application layer protocols transported over
TCP; therefore it is easy to transport commands over non-standard
ports or inject commands into authorized traffic flows.
o Protocol commands may be available that are capable of
broadcasting messages to many devices at once (i.e. a potential
DoS).
o Protocol commands may be available to query the device network to
obtain defined points and their values (i.e. a configuration
scan).
o Protocol commands may be available that will list all available
function codes (i.e. a function scan).
These inherent vulnerabilities, along with increasing connectivity
between IT an OT networks, make network-based attacks very feasible.
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Simple injection of malicious protocol commands provides control over
the target process. Altering legitimate protocol traffic can also
alter information about a process and disrupt the legitimate controls
that are in place over that process. A man-in-the-middle attack
could provide both control over a process and misrepresentation of
data back to operator consoles.
3.3. Electrical Utilities Future
The business and technology trends that are sweeping the utility
industry will drastically transform the utility business from the way
it has been for many decades. At the core of many of these changes
is a drive to modernize the electrical grid with an integrated
telecommunications infrastructure. However, interoperability
concerns, legacy networks, disparate tools, and stringent security
requirements all add complexity to the grid transformation. Given
the range and diversity of the requirements that should be addressed
by the next generation telecommunications infrastructure, utilities
need to adopt a holistic architectural approach to integrate the
electrical grid with digital telecommunications across the entire
power delivery chain.
The key to modernizing grid telecommunications is to provide a
common, adaptable, multi-service network infrastructure for the
entire utility organization. Such a network serves as the platform
for current capabilities while enabling future expansion of the
network to accommodate new applications and services.
To meet this diverse set of requirements, both today and in the
future, the next generation utility telecommunnications network will
be based on open-standards-based IP architecture. An end-to-end IP
architecture takes advantage of nearly three decades of IP technology
development, facilitating interoperability and device management
across disparate networks and devices, as it has been already
demonstrated in many mission-critical and highly secure networks.
IPv6 is seen as a future telecommunications technology for the Smart
Grid; the IEC (International Electrotechnical Commission) and
different National Committees have mandated a specific adhoc group
(AHG8) to define the migration strategy to IPv6 for all the IEC TC57
power automation standards. The AHG8 has recently finalised the work
on the migration strategy and the following Technical Report has been
issued: IEC TR 62357-200:2015: Guidelines for migration from Internet
Protocol version 4 (IPv4) to Internet Protocol version 6 (IPv6).
We expect cloud-based SCADA systems to control and monitor the
critical and non-critical subsystems of generation systems, for
example wind farms.
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Internet-Draft DetNet Use Cases September 20173.3.1. Migration to Packet-Switched Network
Throughout the world, utilities are increasingly planning for a
future based on smart grid applications requiring advanced
telecommunications systems. Many of these applications utilize
packet connectivity for communicating information and control signals
across the utility's Wide Area Network (WAN), made possible by
technologies such as multiprotocol label switching (MPLS). The data
that traverses the utility WAN includes:
o Grid monitoring, control, and protection data
o Non-control grid data (e.g. asset data for condition-based
monitoring)
o Physical safety and security data (e.g. voice and video)
o Remote worker access to corporate applications (voice, maps,
schematics, etc.)
o Field area network backhaul for smart metering, and distribution
grid management
o Enterprise traffic (email, collaboration tools, business
applications)
WANs support this wide variety of traffic to and from substations,
the transmission and distribution grid, generation sites, between
control centers, and between work locations and data centers. To
maintain this rapidly expanding set of applications, many utilities
are taking steps to evolve present time-division multiplexing (TDM)
based and frame relay infrastructures to packet systems. Packet-
based networks are designed to provide greater functionalities and
higher levels of service for applications, while continuing to
deliver reliability and deterministic (real-time) traffic support.
3.3.2. Telecommunications Trends
These general telecommunications topics are in addition to the use
cases that have been addressed so far. These include both current
and future telecommunications related topics that should be factored
into the network architecture and design.
3.3.2.1. General Telecommunications Requirements
o IP Connectivity everywhere
o Monitoring services everywhere and from different remote centers
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sites, and providing FCAPS (Fault, Configuration, Accounting,
Performance, Security) services from centralized network operation
centers.
Going forward, one network will support operation and maintenance of
electrical networks (generation, transmission, and distribution),
voice and data services for ten of thousands of employees and for
exchange with neighboring interconnections, and administrative
services. To meet those requirements, utility may deploy several
physical networks leveraging different technologies across the
country: an optical network and a microwave network for instance.
Each protection and automatism system between two points has two
telecommunications circuits, one on each network. Path diversity
between two substations is key. Regardless of the event type
(hurricane, ice storm, etc.), one path shall stay available so the
system can still operate.
In the optical network, signals are transmitted over more than tens
of thousands of circuits using fiber optic links, microwave and
telephone cables. This network is the nervous system of the
utility's power transmission operations. The optical network
represents ten of thousands of km of cable deployed along the power
lines, with individual runs as long as 280 km.
3.3.2.3. Precision Time Protocol
Some utilities do not use GPS clocks in generation substations. One
of the main reasons is that some of the generation plants are 30 to
50 meters deep under ground and the GPS signal can be weak and
unreliable. Instead, atomic clocks are used. Clocks are
synchronized amongst each other. Rubidium clocks provide clock and
1ms timestamps for IRIG-B.
Some companies plan to transition to the Precision Time Protocol
(PTP, [IEEE1588]), distributing the synchronization signal over the
IP/MPLS network. PTP provides a mechanism for synchronizing the
clocks of participating nodes to a high degree of accuracy and
precision.
PTP operates based on the following assumptions:
It is assumed that the network eliminates cyclic forwarding of PTP
messages within each communication path (e.g. by using a spanning
tree protocol).
PTP is tolerant of an occasional missed message, duplicated
message, or message that arrived out of order. However, PTP
assumes that such impairments are relatively rare.
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PTP was designed assuming a multicast communication model, however
PTP also supports a unicast communication model as long as the
behavior of the protocol is preserved.
Like all message-based time transfer protocols, PTP time accuracy
is degraded by delay asymmetry in the paths taken by event
messages. Asymmetry is not detectable by PTP, however, if such
delays are known a priori, PTP can correct for asymmetry.
IEC 61850 defines the use of IEC/IEEE 61850-9-3:2016. The title is:
Precision time protocol profile for power utility automation. It is
based on Annex B/IEC 62439 which offers the support of redundant
attachment of clocks to Parallel Redundancy Protocol (PRP) and High-
availability Seamless Redundancy (HSR) networks.
3.3.3. Security Trends in Utility Networks
Although advanced telecommunications networks can assist in
transforming the energy industry by playing a critical role in
maintaining high levels of reliability, performance, and
manageability, they also introduce the need for an integrated
security infrastructure. Many of the technologies being deployed to
support smart grid projects such as smart meters and sensors can
increase the vulnerability of the grid to attack. Top security
concerns for utilities migrating to an intelligent smart grid
telecommunications platform center on the following trends:
o Integration of distributed energy resources
o Proliferation of digital devices to enable management, automation,
protection, and control
o Regulatory mandates to comply with standards for critical
infrastructure protection
o Migration to new systems for outage management, distribution
automation, condition-based maintenance, load forecasting, and
smart metering
o Demand for new levels of customer service and energy management
This development of a diverse set of networks to support the
integration of microgrids, open-access energy competition, and the
use of network-controlled devices is driving the need for a converged
security infrastructure for all participants in the smart grid,
including utilities, energy service providers, large commercial and
industrial, as well as residential customers. Securing the assets of
electric power delivery systems (from the control center to the
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substation, to the feeders and down to customer meters) requires an
end-to-end security infrastructure that protects the myriad of
telecommunications assets used to operate, monitor, and control power
flow and measurement.
"Cyber security" refers to all the security issues in automation and
telecommunications that affect any functions related to the operation
of the electric power systems. Specifically, it involves the
concepts of:
o Integrity : data cannot be altered undetectably
o Authenticity : the telecommunications parties involved must be
validated as genuine
o Authorization : only requests and commands from the authorized
users can be accepted by the system
o Confidentiality : data must not be accessible to any
unauthenticated users
When designing and deploying new smart grid devices and
telecommunications systems, it is imperative to understand the
various impacts of these new components under a variety of attack
situations on the power grid. Consequences of a cyber attack on the
grid telecommunications network can be catastrophic. This is why
security for smart grid is not just an ad hoc feature or product,
it's a complete framework integrating both physical and Cyber
security requirements and covering the entire smart grid networks
from generation to distribution. Security has therefore become one
of the main foundations of the utility telecom network architecture
and must be considered at every layer with a defense-in-depth
approach. Migrating to IP based protocols is key to address these
challenges for two reasons:
o IP enables a rich set of features and capabilities to enhance the
security posture
o IP is based on open standards, which allows interoperability
between different vendors and products, driving down the costs
associated with implementing security solutions in OT networks.
Securing OT (Operation technology) telecommunications over packet-
switched IP networks follow the same principles that are foundational
for securing the IT infrastructure, i.e., consideration must be given
to enforcing electronic access control for both person-to-machine and
machine-to-machine communications, and providing the appropriate
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levels of data privacy, device and platform integrity, and threat
detection and mitigation.
3.4. Electrical Utilities Asks
o Mixed L2 and L3 topologies
o Deterministic behavior
o Bounded latency and jitter
o Tight feedback intervals
o High availability, low recovery time
o Redundancy, low packet loss
o Precise timing
o Centralized computing of deterministic paths
o Distributed configuration may also be useful
4. Building Automation Systems4.1. Use Case Description
A Building Automation System (BAS) manages equipment and sensors in a
building for improving residents' comfort, reducing energy
consumption, and responding to failures and emergencies. For
example, the BAS measures the temperature of a room using sensors and
then controls the HVAC (heating, ventilating, and air conditioning)
to maintain a set temperature and minimize energy consumption.
A BAS primarily performs the following functions:
o Periodically measures states of devices, for example humidity and
illuminance of rooms, open/close state of doors, FAN speed, etc.
o Stores the measured data.
o Provides the measured data to BAS systems and operators.
o Generates alarms for abnormal state of devices.
o Controls devices (e.g. turn off room lights at 10:00 PM).
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o Send an alarm signal to operators if it detects abnormal devices
states.
The BMS and HMI communicate with LCs via IP-based "management
protocols" (see standards [bacnetip], [knx]).
A LC is typically a Programmable Logic Controller (PLC) which is
connected to several tens or hundreds of devices using "field
protocols". An LC performs the following kinds of operations:
o Measure device states and provide the information to BMS or HMI.
o Send control values to devices, unilaterally or as part of a
feedback control loop.
There are many field protocols used today; some are standards-based
and others are proprietary (see standards [lontalk], [modbus],
[profibus] and [flnet]). The result is that BASs have multiple MAC/
PHY modules and interfaces. This makes BASs more expensive, slower
to develop, and can result in "vendor lock-in" with multiple types of
management applications.
4.2.2. BAS Deployment Model
An example BAS for medium or large buildings is shown in Figure 5.
The physical layout spans multiple floors, and there is a monitoring
room where the BAS management entities are located. Each floor will
have one or more LCs depending upon the number of devices connected
to the field network.
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+---------------+
| Remote Center |
| |
| BMS HMI |
+------------------------------------+ | | | |
| Floor 2 | | +---+---+ |
| +----LC~~~~+~~~~~+ Field Network| | | |
| | | | | | Router |
| | Dev Dev | +-------|-------+
| | | |
|--- | ------------------------------| |
| | Floor 1 | |
| +----LC~~~~+~~~~~+ | |
| | | | | |
| | Dev Dev | |
| | | |
| | Management Network | WAN |
| +------------------------Router-------------+
| |
+------------------------------------+
Figure 6: Deployment model for Small Buildings
Some interoperability is possible today in the Management Network,
but not in today's field networks due to their non-IP-based design.
4.2.3. Use Cases for Field Networks
Below are use cases for Environmental Monitoring, Fire Detection, and
Feedback Control, and their implications for field network
performance.
4.2.3.1. Environmental Monitoring
The BMS polls each LC at a maximum measurement interval of 100ms (for
example to draw a historical chart of 1 second granularity with a 10x
sampling interval) and then performs the operations as specified by
the operator. Each LC needs to measure each of its several hundred
sensors once per measurement interval. Latency is not critical in
this scenario as long as all sensor values are completed in the
measurement interval. Availability is expected to be 99.999 %.
4.2.3.2. Fire Detection
On detection of a fire, the BMS must stop the HVAC, close the fire
shutters, turn on the fire sprinklers, send an alarm, etc. There are
typically ~10s of sensors per LC that BMS needs to manage. In this
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scenario the measurement interval is 10-50ms, the communication delay
is 10ms, and the availability must be 99.9999 %.
4.2.3.3. Feedback Control
BAS systems utilize feedback control in various ways; the most time-
critial is control of DC motors, which require a short feedback
interval (1-5ms) with low communication delay (10ms) and jitter
(1ms). The feedback interval depends on the characteristics of the
device and a target quality of control value. There are typically
~10s of such devices per LC.
Communication delay is expected to be less than 10 ms, jitter less
than 1 sec while the availability must be 99.9999% .
4.2.4. Security Considerations
When BAS field networks were developed it was assumed that the field
networks would always be physically isolated from external networks
and therefore security was not a concern. In today's world many BASs
are managed remotely and are thus connected to shared IP networks and
so security is definitely a concern, yet security features are not
available in the majority of BAS field network deployments .
The management network, being an IP-based network, has the protocols
available to enable network security, but in practice many BAS
systems do not implement even the available security features such as
device authentication or encryption for data in transit.
4.3. BAS Future
In the future we expect more fine-grained environmental monitoring
and lower energy consumption, which will require more sensors and
devices, thus requiring larger and more complex building networks.
We expect building networks to be connected to or converged with
other networks (Enterprise network, Home network, and Internet).
Therefore better facilities for network management, control,
reliability and security are critical in order to improve resident
and operator convenience and comfort. For example the ability to
monitor and control building devices via the internet would enable
(for example) control of room lights or HVAC from a resident's
desktop PC or phone application.
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The community would like to see an interoperable protocol
specification that can satisfy the timing, security, availability and
QoS constraints described above, such that the resulting converged
network can replace the disparate field networks. Ideally this
connectivity could extend to the open Internet.
This would imply an architecture that can guarantee
o Low communication delays (from <10ms to 100ms in a network of
several hundred devices)
o Low jitter (< 1 ms)
o Tight feedback intervals (1ms - 10ms)
o High network availability (up to 99.9999% )
o Availability of network data in disaster scenario
o Authentication between management and field devices (both local
and remote)
o Integrity and data origin authentication of communication data
between field and management devices
o Confidentiality of data when communicated to a remote device
5. Wireless for Industrial5.1. Use Case Description
Wireless networks are useful for industrial applications, for example
when portable, fast-moving or rotating objects are involved, and for
the resource-constrained devices found in the Internet of Things
(IoT).
Such network-connected sensors, actuators, control loops (etc.)
typically require that the underlying network support real-time
quality of service (QoS), as well as specific classes of other
network properties such as reliability, redundancy, and security.
These networks may also contain very large numbers of devices, for
example for factories, "big data" acquisition, and the IoT. Given
the large numbers of devices installed, and the potential
pervasiveness of the IoT, this is a huge and very cost-sensitive
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market. For example, a 1% cost reduction in some areas could save
$100B
5.1.1. Network Convergence using 6TiSCH
Some wireless network technologies support real-time QoS, and are
thus useful for these kinds of networks, but others do not. For
example WiFi is pervasive but does not provide guaranteed timing or
delivery of packets, and thus is not useful in this context.
In this use case we focus on one specific wireless network technology
which does provide the required deterministic QoS, which is "IPv6
over the TSCH mode of IEEE 802.15.4e" (6TiSCH, where TSCH stands for
"Time-Slotted Channel Hopping", see [I-D.ietf-6tisch-architecture],
[IEEE802154], [IEEE802154e], and [RFC7554]).
There are other deterministic wireless busses and networks available
today, however they are imcompatible with each other, and
incompatible with IP traffic (for example [ISA100], [WirelessHART]).
Thus the primary goal of this use case is to apply 6TiSH as a
converged IP- and standards-based wireless network for industrial
applications, i.e. to replace multiple proprietary and/or
incompatible wireless networking and wireless network management
standards.
5.1.2. Common Protocol Development for 6TiSCH
Today there are a number of protocols required by 6TiSCH which are
still in development, and a second intent of this use case is to
highlight the ways in which these "missing" protocols share goals in
common with DetNet. Thus it is possible that some of the protocol
technology developed for DetNet will also be applicable to 6TiSCH.
These protocol goals are identified here, along with their
relationship to DetNet. It is likely that ultimately the resulting
protocols will not be identical, but will share design principles
which contribute to the eficiency of enabling both DetNet and 6TiSCH.
One such commonality is that although at a different time scale, in
both TSN [IEEE802.1TSNTG] and TSCH a packet crosses the network from
node to node follows a precise schedule, as a train that leaves
intermediate stations at precise times along its path. This kind of
operation reduces collisions, saves energy, and enables engineering
the network for deterministic properties.
Another commonality is remote monitoring and scheduling management of
a TSCH network by a Path Computation Element (PCE) and Network
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Management Entity (NME). The PCE/NME manage timeslots and device
resources in a manner that minimizes the interaction with and the
load placed on resource-constrained devices. For example, a tiny IoT
device may have just enough buffers to store one or a few IPv6
packets, and will have limited bandwidth between peers such that it
can maintain only a small amount of peer information, and will not be
able to store many packets waiting to be forwarded. It is
advantageous then for it to only be required to carry out the
specific behavior assigned to it by the PCE/NME (as opposed to
maintaining its own IP stack, for example).
Note: Current WG discussion indicates that some peer-to-peer
communication must be assumed, i.e. the PCE may communicate only
indirectly with any given device, enabling hierarchical configuration
of the system.
6TiSCH depends on [PCE] and [I-D.finn-detnet-architecture].
6TiSCH also depends on the fact that DetNet will maintain consistency
with [IEEE802.1TSNTG].
5.2. Wireless Industrial Today
Today industrial wireless is accomplished using multiple
deterministic wireless networks which are incompatible with each
other and with IP traffic.
6TiSCH is not yet fully specified, so it cannot be used in today's
applications.
5.3. Wireless Industrial Future5.3.1. Unified Wireless Network and Management
We expect DetNet and 6TiSCH together to enable converged transport of
deterministic and best-effort traffic flows between real-time
industrial devices and wide area networks via IP routing. A high
level view of a basic such network is shown in Figure 7.
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The PCE must compute a deterministic path end-to-end across the TSCH
network and IEEE802.1 TSN Ethernet backbone, and DetNet protocols are
expected to enable end-to-end deterministic forwarding.
+-----+
| IoT |
| G/W |
+-----+
^ <---- Elimination
| |
Track branch | |
+-------+ +--------+ Subnet Backbone
| |
+--|--+ +--|--+
| | | Backbone | | | Backbone
o | | | router | | | router
+--/--+ +--|--+
o / o o---o----/ o
o o---o--/ o o o o o
o \ / o o LLN o
o v <---- Replication
o
Figure 9: 6TiSCH Network with PRE
5.3.1.1. PCE and 6TiSCH ARQ Retries
Note: The possible use of ARQ techniques in DetNet is currently
considered a possible design alternative.
6TiSCH uses the IEEE802.15.4 Automatic Repeat-reQuest (ARQ) mechanism
to provide higher reliability of packet delivery. ARQ is related to
packet replication and elimination because there are two independent
paths for packets to arrive at the destination, and if an expected
packed does not arrive on one path then it checks for the packet on
the second path.
Although to date this mechanism is only used by wireless networks,
this may be a technique that would be appropriate for DetNet and so
aspects of the enabling protocol could be co-developed.
For example, in Figure 9, a Track is laid out from a field device in
a 6TiSCH network to an IoT gateway that is located on a IEEE802.1 TSN
backbone.
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In ARQ the Replication function in the field device sends a copy of
each packet over two different branches, and the PCE schedules each
hop of both branches so that the two copies arrive in due time at the
gateway. In case of a loss on one branch, hopefully the other copy
of the packet still arrives within the allocated time. If two copies
make it to the IoT gateway, the Elimination function in the gateway
ignores the extra packet and presents only one copy to upper layers.
At each 6TiSCH hop along the Track, the PCE may schedule more than
one timeSlot for a packet, so as to support Layer-2 retries (ARQ).
In current deployments, a TSCH Track does not necessarily support PRE
but is systematically multi-path. This means that a Track is
scheduled so as to ensure that each hop has at least two forwarding
solutions, and the forwarding decision is to try the preferred one
and use the other in case of Layer-2 transmission failure as detected
by ARQ.
5.3.2. Schedule Management by a PCE
A common feature of 6TiSCH and DetNet is the action of a PCE to
configure paths through the network. Specifically, what is needed is
a protocol and data model that the PCE will use to get/set the
relevant configuration from/to the devices, as well as perform
operations on the devices. We expect that this protocol will be
developed by DetNet with consideration for its reuse by 6TiSCH. The
remainder of this section provides a bit more context from the 6TiSCH
side.
5.3.2.1. PCE Commands and 6TiSCH CoAP Requests
The 6TiSCH device does not expect to place the request for bandwidth
between itself and another device in the network. Rather, an
operation control system invoked through a human interface specifies
the required traffic specification and the end nodes (in terms of
latency and reliability). Based on this information, the PCE must
compute a path between the end nodes and provision the network with
per-flow state that describes the per-hop operation for a given
packet, the corresponding timeslots, and the flow identification that
enables recognizing that a certain packet belongs to a certain path,
etc.
For a static configuration that serves a certain purpose for a long
period of time, it is expected that a node will be provisioned in one
shot with a full schedule, which incorporates the aggregation of its
behavior for multiple paths. 6TiSCH expects that the programing of
the schedule will be done over COAP as discussed in
[I-D.ietf-6tisch-coap].
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6TiSCH expects that the PCE commands will be mapped back and forth
into CoAP by a gateway function at the edge of the 6TiSCH network.
For instance, it is possible that a mapping entity on the backbone
transforms a non-CoAP protocol such as PCEP into the RESTful
interfaces that the 6TiSCH devices support. This architecture will
be refined to comply with DetNet [I-D.finn-detnet-architecture] when
the work is formalized. Related information about 6TiSCH can be
found at [I-D.ietf-6tisch-6top-interface] and RPL [RFC6550].
A protocol may be used to update the state in the devices during
runtime, for example if it appears that a path through the network
has ceased to perform as expected, but in 6TiSCH that flow was not
designed and no protocol was selected. We would like to see DetNet
define the appropriate end-to-end protocols to be used in that case.
The implication is that these state updates take place once the
system is configured and running, i.e. they are not limited to the
initial communication of the configuration of the system.
A "slotFrame" is the base object that a PCE would manipulate to
program a schedule into an LLN node ([I-D.ietf-6tisch-architecture]).
We would like to see the PCE read energy data from devices, and
compute paths that will implement policies on how energy in devices
is consumed, for instance to ensure that the spent energy does not
exceeded the available energy over a period of time. Note: this
statement implies that an extensible protocol for communicating
device info to the PCE and enabling the PCE to act on it will be part
of the DetNet architecture, however for subnets with specific
protocols (e.g. CoAP) a gateway may be required.
6TiSCH devices can discover their neighbors over the radio using a
mechanism such as beacons, but even though the neighbor information
is available in the 6TiSCH interface data model, 6TiSCH does not
describe a protocol to proactively push the neighborhood information
to a PCE. We would like to see DetNet define such a protocol; one
possible design alternative is that it could operate over CoAP,
alternatively it could be converted to/from CoAP by a gateway. We
would like to see such a protocol carry multiple metrics, for example
similar to those used for RPL operations [RFC6551]
5.3.2.2. 6TiSCH IP Interface
"6top" ([I-D.wang-6tisch-6top-sublayer]) is a logical link control
sitting between the IP layer and the TSCH MAC layer which provides
the link abstraction that is required for IP operations. The 6top
data model and management interfaces are further discussed in
[I-D.ietf-6tisch-6top-interface] and [I-D.ietf-6tisch-coap].
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An IP packet that is sent along a 6TiSCH path uses the Differentiated
Services Per-Hop-Behavior Group called Deterministic Forwarding, as
described in [I-D.svshah-tsvwg-deterministic-forwarding].
5.3.3. 6TiSCH Security Considerations
On top of the classical requirements for protection of control
signaling, it must be noted that 6TiSCH networks operate on limited
resources that can be depleted rapidly in a DoS attack on the system,
for instance by placing a rogue device in the network, or by
obtaining management control and setting up unexpected additional
paths.
5.4. Wireless Industrial Asks
6TiSCH depends on DetNet to define:
o Configuration (state) and operations for deterministic paths
o End-to-end protocols for deterministic forwarding (tagging, IP)
o Protocol for packet replication and elimination
6. Cellular Radio6.1. Use Case Description
This use case describes the application of deterministic networking
in the context of cellular telecom transport networks. Important
elements include time synchronization, clock distribution, and ways
of establishing time-sensitive streams for both Layer-2 and Layer-3
user plane traffic.
6.1.1. Network Architecture
Figure 10 illustrates a typical 3GPP-defined cellular network
architecture, which includes "Fronthaul", "Midhaul" and "Backhaul"
network segments. The "Fronthaul" is the network connecting base
stations (baseband processing units) to the remote radio heads
(antennas). The "Midhaul" is the network inter-connecting base
stations (or small cell sites). The "Backhaul" is the network or
links connecting the radio base station sites to the network
controller/gateway sites (i.e. the core of the 3GPP cellular
network).
In Figure 10 "eNB" ("E-UTRAN Node B") is the hardware that is
connected to the mobile phone network which communicates directly
with mobile handsets ([TS36300]).
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Y (remote radio heads (antennas))
\
Y__ \.--. .--. +------+
\_( `. +---+ _(Back`. | 3GPP |
Y------( Front )----|eNB|----( Haul )----| core |
( ` .Haul ) +---+ ( ` . ) ) | netw |
/`--(___.-' \ `--(___.-' +------+
Y_/ / \.--. \
Y_/ _( Mid`. \
( Haul ) \
( ` . ) ) \
`--(___.-'\_____+---+ (small cell sites)
\ |SCe|__Y
+---+ +---+
Y__|eNB|__Y
+---+
Y_/ \_Y ("local" radios)
Figure 10: Generic 3GPP-based Cellular Network Architecture
6.1.2. Delay Constraints
The available processing time for Fronthaul networking overhead is
limited to the available time after the baseband processing of the
radio frame has completed. For example in Long Term Evolution (LTE)
radio, processing of a radio frame is allocated 3ms but typically the
processing uses most of it, allowing only a small fraction to be used
by the Fronthaul network (e.g. up to 250us one-way delay, though the
existing spec ([NGMN-fronth]) supports delay only up to 100us). This
ultimately determines the distance the remote radio heads can be
located from the base stations (e.g., 100us equals roughly 20 km of
optical fiber-based transport). Allocation options of the available
time budget between processing and transport are under heavy
discussions in the mobile industry.
For packet-based transport the allocated transport time (e.g. CPRI
would allow for 100us delay [CPRI]) is consumed by all nodes and
buffering between the remote radio head and the baseband processing
unit, plus the distance-incurred delay.
The baseband processing time and the available "delay budget" for the
fronthaul is likely to change in the forthcoming "5G" due to reduced
radio round trip times and other architectural and service
requirements [NGMN].
The transport time budget, as noted above, places limitations on the
distance that remote radio heads can be located from base stations
(i.e. the link length). In the above analysis, the entire transport
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time budget is assumed to be available for link propagation delay.
However the transport time budget can be broken down into three
components: scheduling /queueing delay, transmission delay, and link
propagation delay. Using today's Fronthaul networking technology,
the queuing, scheduling and transmission components might become the
dominant factors in the total transport time rather than the link
propagation delay. This is especially true in cases where the
Fronthaul link is relatively short and it is shared among multiple
Fronthaul flows, for example in indoor and small cell networks,
massive MIMO antenna networks, and split Fronthaul architectures.
DetNet technology can improve this application by controlling and
reducing the time required for the queuing, scheduling and
transmission operations by properly assigning the network resources,
thus leaving more of the transport time budget available for link
propagation, and thus enabling longer link lengths. However, link
length is usually a given parameter and is not a controllable network
parameter, since RRH and BBU sights are usually located in
predetermined locations. However, the number of antennas in an RRH
sight might increase for example by adding more antennas, increasing
the MIMO capability of the network or support of massive MIMO. This
means increasing the number of the fronthaul flows sharing the same
fronthaul link. DetNet can now control the bandwidth assignment of
the fronthaul link and the scheduling pf fronthaul packets over this
link and provide adequate buffer provisioning for each flow to reduce
the packet loss rate.
Another way in which DetNet technology can aid Fronthaul networks is
by providing effective isolation from best-effort (and other classes
of) traffic, which can arise as a result of network slicing in 5G
networks where Fronthaul traffic generated in different network
slices might have differing performance requirements. DetNet
technology can also dynamically control the bandwidth assignment,
scheduling and packet forwarding decisions and the buffer
provisioning of the Fronthaul flows to guarantee the end-to-end delay
of the Fronthaul packets and minimize the packet loss rate.
[METIS] documents the fundamental challenges as well as overall
technical goals of the future 5G mobile and wireless system as the
starting point. These future systems should support much higher data
volumes and rates and significantly lower end-to-end latency for 100x
more connected devices (at similar cost and energy consumption levels
as today's system).
For Midhaul connections, delay constraints are driven by Inter-Site
radio functions like Coordinated Multipoint Processing (CoMP, see
[CoMP]). CoMP reception and transmission is a framework in which
multiple geographically distributed antenna nodes cooperate to
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improve the performance of the users served in the common cooperation
area. The design principal of CoMP is to extend the current single-
cell to multi-UE (User Equipment) transmission to a multi-cell-to-
multi-UEs transmission by base station cooperation.
CoMP has delay-sensitive performance parameters, which are "midhaul
latency" and "CSI (Channel State Information) reporting and
accuracy". The essential feature of CoMP is signaling between eNBs,
so Midhaul latency is the dominating limitation of CoMP performance.
Generally, CoMP can benefit from coordinated scheduling (either
distributed or centralized) of different cells if the signaling delay
between eNBs is within 1-10ms. This delay requirement is both rigid
and absolute because any uncertainty in delay will degrade the
performance significantly.
Inter-site CoMP is one of the key requirements for 5G and is also a
near-term goal for the current 4.5G network architecture.
6.1.3. Time Synchronization Constraints
Fronthaul time synchronization requirements are given by [TS25104],
[TS36104], [TS36211], and [TS36133]. These can be summarized for the
current 3GPP LTE-based networks as:
Delay Accuracy:
+-8ns (i.e. +-1/32 Tc, where Tc is the UMTS Chip time of 1/3.84
MHz) resulting in a round trip accuracy of +-16ns. The value is
this low to meet the 3GPP Timing Alignment Error (TAE) measurement
requirements. Note: performance guarantees of low nanosecond
values such as these are considered to be below the DetNet layer -
it is assumed that the underlying implementation, e.g. the
hardware, will provide sufficient support (e.g. buffering) to
enable this level of accuracy. These values are maintained in the
use case to give an indication of the overall application.
Timing Alignment Error:
Timing Alignment Error (TAE) is problematic to Fronthaul networks
and must be minimized. If the transport network cannot guarantee
low enough TAE then additional buffering has to be introduced at
the edges of the network to buffer out the jitter. Buffering is
not desirable as it reduces the total available delay budget.
Packet Delay Variation (PDV) requirements can be derived from TAE
for packet based Fronthaul networks.
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* For multiple input multiple output (MIMO) or TX diversity
transmissions, at each carrier frequency, TAE shall not exceed
65 ns (i.e. 1/4 Tc).
* For intra-band contiguous carrier aggregation, with or without
MIMO or TX diversity, TAE shall not exceed 130 ns (i.e. 1/2
Tc).
* For intra-band non-contiguous carrier aggregation, with or
without MIMO or TX diversity, TAE shall not exceed 260 ns (i.e.
one Tc).
* For inter-band carrier aggregation, with or without MIMO or TX
diversity, TAE shall not exceed 260 ns.
Transport link contribution to radio frequency error:
+-2 PPB. This value is considered to be "available" for the
Fronthaul link out of the total 50 PPB budget reserved for the
radio interface. Note: the reason that the transport link
contributes to radio frequency error is as follows. The current
way of doing Fronthaul is from the radio unit to remote radio head
directly. The remote radio head is essentially a passive device
(without buffering etc.) The transport drives the antenna
directly by feeding it with samples and everything the transport
adds will be introduced to radio as-is. So if the transport
causes additional frequency error that shows immediately on the
radio as well. Note: performance guarantees of low nanosecond
values such as these are considered to be below the DetNet layer -
it is assumed that the underlying implementation, e.g. the
hardware, will provide sufficient support to enable this level of
performance. These values are maintained in the use case to give
an indication of the overall application.
The above listed time synchronization requirements are difficult to
meet with point-to-point connected networks, and more difficult when
the network includes multiple hops. It is expected that networks
must include buffering at the ends of the connections as imposed by
the jitter requirements, since trying to meet the jitter requirements
in every intermediate node is likely to be too costly. However,
every measure to reduce jitter and delay on the path makes it easier
to meet the end-to-end requirements.
In order to meet the timing requirements both senders and receivers
must remain time synchronized, demanding very accurate clock
distribution, for example support for IEEE 1588 transparent clocks or
boundary clocks in every intermediate node.
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In cellular networks from the LTE radio era onward, phase
synchronization is needed in addition to frequency synchronization
([TS36300], [TS23401]). Time constraints are also important due to
their impact on packet loss. If a packet is delivered too late, then
the packet may be dropped by the host.
6.1.4. Transport Loss Constraints
Fronthaul and Midhaul networks assume almost error-free transport.
Errors can result in a reset of the radio interfaces, which can cause
reduced throughput or broken radio connectivity for mobile customers.
For packetized Fronthaul and Midhaul connections packet loss may be
caused by BER, congestion, or network failure scenarios. Different
fronthaul functional splits are being considered by 3GPP, requiring
strict frame loss ratio (FLR) guarantees. As one example (referring
to the legacy CPRI split which is option 8 in 3GPP) lower layers
splits may imply an FLR of less than 10E-7 for data traffic and less
than 10E-6 for control and management traffic. Current tools for
eliminating packet loss for Fronthaul and Midhaul networks have
serious challenges, for example retransmitting lost packets and/or
using forward error correction (FEC) to circumvent bit errors is
practically impossible due to the additional delay incurred. Using
redundant streams for better guarantees for delivery is also
practically impossible in many cases due to high bandwidth
requirements of Fronthaul and Midhaul networks. Protection switching
is also a candidate but current technologies for the path switch are
too slow to avoid reset of mobile interfaces.
Fronthaul links are assumed to be symmetric, and all Fronthaul
streams (i.e. those carrying radio data) have equal priority and
cannot delay or pre-empt each other. This implies that the network
must guarantee that each time-sensitive flow meets their schedule.
6.1.5. Security Considerations
Establishing time-sensitive streams in the network entails reserving
networking resources for long periods of time. It is important that
these reservation requests be authenticated to prevent malicious
reservation attempts from hostile nodes (or accidental
misconfiguration). This is particularly important in the case where
the reservation requests span administrative domains. Furthermore,
the reservation information itself should be digitally signed to
reduce the risk of a legitimate node pushing a stale or hostile
configuration into another networking node.
Note: This is considered important for the security policy of the
network, but does not affect the core DetNet architecture and design.
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Internet-Draft DetNet Use Cases September 20176.2. Cellular Radio Networks Today6.2.1. Fronthaul
Today's Fronthaul networks typically consist of:
o Dedicated point-to-point fiber connection is common
o Proprietary protocols and framings
o Custom equipment and no real networking
Current solutions for Fronthaul are direct optical cables or
Wavelength-Division Multiplexing (WDM) connections.
6.2.2. Midhaul and Backhaul
Today's Midhaul and Backhaul networks typically consist of:
o Mostly normal IP networks, MPLS-TP, etc.
o Clock distribution and sync using 1588 and SyncE
Telecommunication networks in the Mid- and Backhaul are already
heading towards transport networks where precise time synchronization
support is one of the basic building blocks. While the transport
networks themselves have practically transitioned to all-IP packet-
based networks to meet the bandwidth and cost requirements, highly
accurate clock distribution has become a challenge.
In the past, Mid- and Backhaul connections were typically based on
Time Division Multiplexing (TDM-based) and provided frequency
synchronization capabilities as a part of the transport media.
Alternatively other technologies such as Global Positioning System
(GPS) or Synchronous Ethernet (SyncE) are used [SyncE].
Both Ethernet and IP/MPLS [RFC3031] (and PseudoWires (PWE) [RFC3985]
for legacy transport support) have become popular tools to build and
manage new all-IP Radio Access Networks (RANs)
[I-D.kh-spring-ip-ran-use-case]. Although various timing and
synchronization optimizations have already been proposed and
implemented including 1588 PTP enhancements
[I-D.ietf-tictoc-1588overmpls] and [I-D.ietf-mpls-residence-time],
these solution are not necessarily sufficient for the forthcoming RAN
architectures nor do they guarantee the more stringent time-
synchronization requirements such as [CPRI].
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There are also existing solutions for TDM over IP such as [RFC5087]
and [RFC4553], as well as TDM over Ethernet transports such as
[RFC5086].
6.3. Cellular Radio Networks Future
Future Cellular Radio Networks will be based on a mix of different
xHaul networks (xHaul = front-, mid- and backhaul), and future
transport networks should be able to support all of them
simultaneously. It is already envisioned today that:
o Not all "cellular radio network" traffic will be IP, for example
some will remain at Layer 2 (e.g. Ethernet based). DetNet
solutions must address all traffic types (Layer 2, Layer 3) with
the same tools and allow their transport simultaneously.
o All forms of xHaul networks will need some form of DetNet
solutions. For example with the advent of 5G some Backhaul
traffic will also have DetNet requirements, for example traffic
belonging to time-critical 5G applications.
o Different splits of the functionality run on the base stations and
the on-site units could co-exist on the same Fronthaul and
Backhaul network.
We would like to see the following in future Cellular Radio networks:
o Unified standards-based transport protocols and standard
networking equipment that can make use of underlying deterministic
link-layer services
o Unified and standards-based network management systems and
protocols in all parts of the network (including Fronthaul)
New radio access network deployment models and architectures may
require time- sensitive networking services with strict requirements
on other parts of the network that previously were not considered to
be packetized at all. Time and synchronization support are already
topical for Backhaul and Midhaul packet networks [MEF] and are
becoming a real issue for Fronthaul networks also. Specifically in
Fronthaul networks the timing and synchronization requirements can be
extreme for packet based technologies, for example, on the order of
sub +-20 ns packet delay variation (PDV) and frequency accuracy of
+0.002 PPM [Fronthaul].
The actual transport protocols and/or solutions to establish required
transport "circuits" (pinned-down paths) for Fronthaul traffic are
still undefined. Those are likely to include (but are not limited
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to) solutions directly over Ethernet, over IP, and using MPLS/
PseudoWire transport.
Even the current time-sensitive networking features may not be
sufficient for Fronthaul traffic. Therefore, having specific
profiles that take the requirements of Fronthaul into account is
desirable [IEEE8021CM].
Interesting and important work for time-sensitive networking has been
done for Ethernet [TSNTG], which specifies the use of IEEE 1588 time
precision protocol (PTP) [IEEE1588] in the context of IEEE 802.1D and
IEEE 802.1Q. [IEEE8021AS] specifies a Layer 2 time synchronizing
service, and other specifications such as IEEE 1722 [IEEE1722]
specify Ethernet-based Layer-2 transport for time-sensitive streams.
New promising work seeks to enable the transport of time-sensitive
fronthaul streams in Ethernet bridged networks [IEEE8021CM].
Analogous to IEEE 1722 there is an ongoing standardization effort to
define the Layer-2 transport encapsulation format for transporting
radio over Ethernet (RoE) in the IEEE 1904.3 Task Force [IEEE19043].
All-IP RANs and xHhaul networks would benefit from time
synchronization and time-sensitive transport services. Although
Ethernet appears to be the unifying technology for the transport,
there is still a disconnect providing Layer 3 services. The protocol
stack typically has a number of layers below the Ethernet Layer 2
that shows up to the Layer 3 IP transport. It is not uncommon that
on top of the lowest layer (optical) transport there is the first
layer of Ethernet followed one or more layers of MPLS, PseudoWires
and/or other tunneling protocols finally carrying the Ethernet layer
visible to the user plane IP traffic.
While there are existing technologies to establish circuits through
the routed and switched networks (especially in MPLS/PWE space),
there is still no way to signal the time synchronization and time-
sensitive stream requirements/reservations for Layer-3 flows in a way
that addresses the entire transport stack, including the Ethernet
layers that need to be configured.
Furthermore, not all "user plane" traffic will be IP. Therefore, the
same solution also must address the use cases where the user plane
traffic is a different layer, for example Ethernet frames.
There is existing work describing the problem statement
[I-D.finn-detnet-problem-statement] and the architecture
[I-D.finn-detnet-architecture] for deterministic networking (DetNet)
that targets solutions for time-sensitive (IP/transport) streams with
deterministic properties over Ethernet-based switched networks.
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A standard for data plane transport specification which is:
o Unified among all xHauls (meaning that different flows with
diverse DetNet requirements can coexist in the same network and
traverse the same nodes without interfering with each other)
o Deployed in a highly deterministic network environment
o Capable of supporting multiple functional splits simultaneously,
including existing Backhaul and CPRI Fronthaul and potentially new
modes as defined for example in 3GPP; these goals can be supported
by the existing DetNet Use Case Common Themes, notably "Mix of
Deterministic and Best-Effort Traffic", "Bounded Latency", "Low
Latency", "Symmetrical Path Delays", and "Deterministic Flows".
o Capable of supporting Network Slicing and Multi-tenancy; these
goals can be supported by the same DetNet themes noted above.
o Capable of transporting both in-band and out-band control traffic
(OAM info, ...).
o Deployable over multiple data link technologies (e.g., IEEE 802.3,
mmWave, etc.).
A standard for data flow information models that are:
o Aware of the time sensitivity and constraints of the target
networking environment
o Aware of underlying deterministic networking services (e.g., on
the Ethernet layer)
7. Industrial M2M7.1. Use Case Description
Industrial Automation in general refers to automation of
manufacturing, quality control and material processing. In this
"machine to machine" (M2M) use case we consider machine units in a
plant floor which periodically exchange data with upstream or
downstream machine modules and/or a supervisory controller within a
local area network.
The actors of M2M communication are Programmable Logic Controllers
(PLCs). Communication between PLCs and between PLCs and the
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supervisory PLC (S-PLC) is achieved via critical control/data streams
Figure 11.
S (Sensor)
\ +-----+
PLC__ \.--. .--. ---| MES |
\_( `. _( `./ +-----+
A------( Local )-------------( L2 )
( Net ) ( Net ) +-------+
/`--(___.-' `--(___.-' ----| S-PLC |
S_/ / PLC .--. / +-------+
A_/ \_( `.
(Actuator) ( Local )
( Net )
/`--(___.-'\
/ \ A
S A
Figure 11: Current Generic Industrial M2M Network Architecture
This use case focuses on PLC-related communications; communication to
Manufacturing-Execution-Systems (MESs) are not addressed.
This use case covers only critical control/data streams; non-critical
traffic between industrial automation applications (such as
communication of state, configuration, set-up, and database
communication) are adequately served by currently available
prioritizing techniques. Such traffic can use up to 80% of the total
bandwidth required. There is also a subset of non-time-critical
traffic that must be reliable even though it is not time sensitive.
In this use case the primary need for deterministic networking is to
provide end-to-end delivery of M2M messages within specific timing
constraints, for example in closed loop automation control. Today
this level of determinism is provided by proprietary networking
technologies. In addition, standard networking technologies are used
to connect the local network to remote industrial automation sites,
e.g. over an enterprise or metro network which also carries other
types of traffic. Therefore, flows that should be forwarded with
deterministic guarantees need to be sustained regardless of the
amount of other flows in those networks.
7.2. Industrial M2M Communication Today
Today, proprietary networks fulfill the needed timing and
availability for M2M networks.
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The network topologies used today by industrial automation are
similar to those used by telecom networks: Daisy Chain, Ring, Hub and
Spoke, and Comb (a subset of Daisy Chain).
PLC-related control/data streams are transmitted periodically and
carry either a pre-configured payload or a payload configured during
runtime.
Some industrial applications require time synchronization at the end
nodes. For such time-coordinated PLCs, accuracy of 1 microsecond is
required. Even in the case of "non-time-coordinated" PLCs time sync
may be needed e.g. for timestamping of sensor data.
Industrial network scenarios require advanced security solutions.
Many of the current industrial production networks are physically
separated. Preventing critical flows from be leaked outside a domain
is handled today by filtering policies that are typically enforced in
firewalls.
7.2.1. Transport Parameters
The Cycle Time defines the frequency of message(s) between industrial
actors. The Cycle Time is application dependent, in the range of 1ms
- 100ms for critical control/data streams.
Because industrial applications assume deterministic transport for
critical Control-Data-Stream parameters (instead of defining latency
and delay variation parameters) it is sufficient to fulfill the upper
bound of latency (maximum latency). The underlying networking
infrastructure must ensure a maximum end-to-end delivery time of
messages in the range of 100 microseconds to 50 milliseconds
depending on the control loop application.
The bandwidth requirements of control/data streams are usually
calculated directly from the bytes-per-cycle parameter of the control
loop. For PLC-to-PLC communication one can expect 2 - 32 streams
with packet size in the range of 100 - 700 bytes. For S-PLC to PLCs
the number of streams is higher - up to 256 streams. Usually no more
than 20% of available bandwidth is used for critical control/data
streams. In today's networks 1Gbps links are commonly used.
Most PLC control loops are rather tolerant of packet loss, however
critical control/data streams accept no more than 1 packet loss per
consecutive communication cycle (i.e. if a packet gets lost in cycle
"n", then the next cycle ("n+1") must be lossless). After two or
more consecutive packet losses the network may be considered to be
"down" by the Application.
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As network downtime may impact the whole production system the
required network availability is rather high (99,999%).
Based on the above parameters we expect that some form of redundancy
will be required for M2M communications, however any individual
solution depends on several parameters including cycle time, delivery
time, etc.
7.2.2. Stream Creation and Destruction
In an industrial environment, critical control/data streams are
created rather infrequently, on the order of ~10 times per day / week
/ month. Most of these critical control/data streams get created at
machine startup, however flexibility is also needed during runtime,
for example when adding or removing a machine. Going forward as
production systems become more flexible, we expect a significant
increase in the rate at which streams are created, changed and
destroyed.
7.3. Industrial M2M Future
We would like to see a converged IP-standards-based network with
deterministic properties that can satisfy the timing, security and
reliability constraints described above. Today's proprietary
networks could then be interfaced to such a network via gateways or,
in the case of new installations, devices could be connected directly
to the converged network.
For this use case we expect time synchronization accuracy on the
order of 1us.
7.4. Industrial M2M Asks
o Converged IP-based network
o Deterministic behavior (bounded latency and jitter )
o High availability (presumably through redundancy) (99.999 %)
o Low message delivery time (100us - 50ms)
o Low packet loss (burstless, 0.1-1 %)
o Security (e.g. prevent critical flows from being leaked between
physically separated networks)
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Internet-Draft DetNet Use Cases September 20178. Mining Industry8.1. Use Case Description
The mining industry is highly dependent on networks to monitor and
control their systems both in open-pit and underground extraction,
transport and refining processes. In order to reduce risks and
increase operational efficiency in mining operations, a number of
processes have migrated the operators from the extraction site to
remote control and monitoring.
In the case of open pit mining, autonomous trucks are used to
transport the raw materials from the open pit to the refining factory
where the final product (e.g. Copper) is obtained. Although the
operation is autonomous, the tracks are remotely monitored from a
central facility.
In pit mines, the monitoring of the tailings or mine dumps is
critical in order to avoid any environmental pollution. In the past,
monitoring has been conducted through manual inspection of pre-
installed dataloggers. Cabling is not usually exploited in such
scenarios due to the cost and complex deployment requirements.
Currently, wireless technologies are being employed to monitor these
cases permanently. Slopes are also monitored in order to anticipate
possible mine collapse. Due to the unstable terrain, cable
maintenance is costly and complex and hence wireless technologies are
employed.
In the underground monitoring case, autonomous vehicles with
extraction tools travel autonomously through the tunnels, but their
operational tasks (such as excavation, stone breaking and transport)
are controlled remotely from a central facility. This generates
video and feedback upstream traffic plus downstream actuator control
traffic.
8.2. Mining Industry Today
Currently the mining industry uses a packet switched architecture
supported by high speed ethernet. However in order to achieve the
delay and packet loss requirements the network bandwidth is
overestimated, thus providing very low efficiency in terms of
resource usage.
QoS is implemented at the Routers to separate video, management,
monitoring and process control traffic for each stream.
Since mobility is involved in this process, the connection between
the backbone and the mobile devices (e.g. trucks, trains and
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excavators) is solved using a wireless link. These links are based
on 802.11 for open-pit mining and leaky feeder for underground
mining.
Lately in pit mines the use of LPWAN technologies has been extended:
Tailings, slopes and mine dumps are monitored by battery-powered
dataloggers that make use of robust long range radio technologies.
Reliability is usually ensured through retransmissions at L2.
Gateways or concentrators act as bridges forwarding the data to the
backbone ethernet network. Deterministic requirements are biased
towards reliability rather than latency as events are slowly
triggered or can be anticipated in advance.
At the mineral processing stage, conveyor belts and refining
processes are controlled by a SCADA system, which provides the in-
factory delay-constrained networking requirements.
Voice communications are currently served by a redundant trunking
infrastructure, independent from current data networks.
8.3. Mining Industry Future
Mining operations and management are currently converging towards a
combination of autonomous operation and teleoperation of transport
and extraction machines. This means that video, audio, monitoring
and process control traffic will increase dramatically. Ideally, all
activities on the mine will rely on network infrastructure.
Wireless for open-pit mining is already a reality with LPWAN
technologies and it is expected to evolve to more advanced LPWAN
technologies such as those based on LTE to increase last hop
reliability or novel LPWAN flavours with deterministic access.
One area in which DetNet can improve this use case is in the wired
networks that make up the "backbone network" of the system, which
connect together many wireless access points (APs). The mobile
machines (which are connected to the network via wireless) transition
from one AP to the next as they move about. A deterministic,
reliable, low latency backbone can enable these transitions to be
more reliable.
Connections which extend all the way from the base stations to the
machinery via a mix of wired and wireless hops would also be
beneficial, for example to improve remote control responsiveness of
digging machines. However to guarantee deterministic performance of
a DetNet, the end-to-end underlying network must be deterministic.
Thus for this use case if a deterministic wireless transport is
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integrated with a wire-based DetNet network, it could create the
desired wired plus wireless end-to-end deterministic network.
8.4. Mining Industry Asks
o Improved bandwidth efficiency
o Very low delay to enable machine teleoperation
o Dedicated bandwidth usage for high resolution video streams
o Predictable delay to enable realtime monitoring
o Potential to construct a unified DetNet network over a combination
of wired and deterministic wireless links
9. Private Blockchain9.1. Use Case Description
Blockchain was created with bitcoin, as a 'public' blockchain on the
open Internet, however blockchain has also spread far beyond its
original host into various industries such as smart manufacturing,
logistics, security, legal rights and others. In these industries
blockchain runs in designated and carefully managed network in which
deterministic networking requirements could be addressed by Detnet.
Such implementations are referred to as 'private' blockchain.
The sole distinction between public and private blockchain is related
to who is allowed to participate in the network, execute the
consensus protocol and maintain the shared ledger.
Today's networks treat the traffic from blockchain on a best-effort
basis, but blockchain operation could be made much more efficient if
deterministic networking service were available to minimize latency
and packet loss in the network.
9.1.1. Blockchain Operation
A 'block' runs as a container of a batch of primary items such as
transactions, property records etc. The blocks are chained in such a
way that the hash of the previous block works as the pointer header
of the new block, where confirmation of each block requires a
consensus mechanism. When an item arrives at a blockchain node, the
latter broadcasts this item to the rest of nodes which receive and
verify it and put it in the ongoing block. Block confirmation
process begins as the amount of items reaches the predefined block
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capacity, and the node broadcasts its proved block to the rest of
nodes to be verified and chained.
9.1.2. Blockchain Network Architecture
Blockchain node communication and coordination is achieved mainly
through frequent point to multi-point communication, however
persistent point-to-point connections are used to transport both the
items and the blocks to the other nodes.
When a node initiates, it first requests the other nodes' address
from a specific entity such as DNS, then it creates persistent
connections each of with other nodes. If node A confirms an item, it
sends the item to the other nodes via the persistent connections.
As a new block in a node completes and gets proved among the nodes,
it starts propagating this block towards its neighbor nodes. Assume
node A receives a block, it sends invite message after verification
to its neighbor B, B checks if the designated block is available, it
responds get message to A if it is unavailable, and A send the
complete block to B. B repeats the process as A to start the next
round of block propagation.
The challenge of blockchain network operation is not overall data
rates, since the volume from both block and item stays between
hundreds of bytes to a couple of mega bytes per second, but is in
transporting the blocks with minimum latency to maximize efficiency
of the blockchain consensus process.
9.1.3. Security Considerations
Security is crucial to blockchain applications, and todayt blockchain
addresses its security issues mainly at the application level, where
cryptography as well as hash-based consensus play a leading role
preventing both double-spending and malicious service attack.
However, there is concern that in the proposed use case of a private
blockchain network which is dependent on deterministic properties,
the network could be vulnerable to delays and other specific attacks
against determinism which could interrupt service.
9.2. Private Blockchain Today
Today private blockchain runs in L2 or L3 VPN, in general without
guaranteed determinism. The industry players are starting to realize
that improving determinism in their blockchain networks could improve
the performance of their service, but as of today these goals are not
being met.
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Internet-Draft DetNet Use Cases September 20179.3. Private Blockchain Future
Blockchain system performance can be greatly improved through
deterministic networking service primarily because it would
accelerate the consensus process. It would be valuable to be able to
design a private blockchain network with the following properties:
o Transport of point to multi-point traffic in a coordinated network
architecture rather than at the application layer (which typically
uses point-to-point connections)
o Guaranteed transport latency
o Reduced packet loss (to the point where packet retransmission-
incurred delay would be negligible.)
9.4. Private Blockchain Asks
o Layer 2 and Layer 3 multicast of blockchain traffic
o Item and block delivery with bounded, low latency and negligible
packet loss
o Coexistence in a single network of blockchain and IT traffic.
o Ability to scale the network by distributing the centralized
control of the network across multiple control entities.
10. Network Slicing10.1. Use Case Description
Network slicing divides one physical network infrastructure into
multiple logical networks. Each slice, corresponding to a logical
network, uses resources and network functions independently from each
other. Network slicing provides flexibility of resource allocation
and service quality customization.
Future services will demand network performance with a wide variety
of characteristics such as high data rate, low latency, low loss
rate, security and many other parameters. Ideally every service
would have its own physical network satisfying its particular
performance requirements, however that would be prohibitively
expensive. Network slicing can provide a customized slice for a
single service, and multiple slices can share the same physical
network. This method can optimize the performance for the service at
lower cost, and the flexibility of setting up and release the slices
also allows the user to allocate the network resources dynamically.
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Unlike other DetNet use cases, Network slicing is not a specific
application with specific deterministic requirements; it is proposed
as a new requirement for the future network, which is still in
discussion, and DetNet is a candidate solution for it.
10.2. Network Slicing Use Cases
Network Slicing is a core feature of 5G defined in 3GPP, which is
currently under development. A Network Slice in mobile network is a
complete logical network including Radio Access Network (RAN) and
Core Network (CN). It provides telecommunication services and
network capabilities, which may vary (or not) from slice to slice.
A 5G bearer network is a typical use case of network slicing,
including 3 service scenarios: enhanced Mobile Broadband (eMBB),
Ultra-Reliable and Low Latency Communications (URLLC), and massive
Machine Type Communications (mMTC). Each of these are described
below.
10.2.1. Enhanced Mobile Broadband (eMBB)
eMBB focuses on services characterized by high data rates, such as
high definition (HD) videos, virtual reality (VR), augmented reality
(AR), and fixed mobile convergence (FMC).
10.2.2. Ultra-Reliable and Low Latency Communications (URLLC)
URLLC focuses on latency-sensitive services, such as self-driving
vehicles, remote surgery, or drone control.
10.2.3. massive Machine Type Communications (mMTC)
mMTC focuses on services that have high requirements for connection
density, such as those typical for smart city and smart agriculture
use cases.
10.3. Using DetNet in Network Slicing
One of the requirements discussed for network slicing is the "hard"
separation of various users' deterministic performance. That is, it
should be impossible for activity, lack of activity, or changes in
activity of one or more users to have any appreciable effect on the
deterministic performance parameters of any other users. Typical
techniques used today, which share a physical network among users, do
not offer this kind of insulation. DetNet can supply point-to-point
or point-to-multipoint paths that offer bandwidth and latency
guarantees to a user that cannot be affected by other users' data
traffic.
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Thus DetNet is a powerful tool when latency and reliability are
required in Network Slicing. However, DetNet cannot cover every
Network Slicing use case, and there are some other problems to be
solved. Firstly, DetNet is a point-to-point or point-to-multipoint
technology while Network Slicing needs multi-point to multi-point
guarantee. Second, the number of flows that can be carried by DetNet
is limited by DetNet scalability. Flow aggregation and queuing
management modification may help to fix the problem. More work and
discussions are needed in these topics.
10.4. Network Slicing Today and Future
Network slicing can satisfy the requirements of a lot of future
deployment scenario, but it is still a collection of ideas and
analysis, without a specific technical solution. A lot of
technologies, such as Flex-E, Segment Routing, and DetNet have
potential to be used in Network Slicing. For more details please see
IETF99 Network Slicing BOF session agenda and materials.
10.5. Network Slicing Asks
o Isolation from other flows through Queuing Management
o Service Quality Customization and Guarantee
o Security
11. Use Case Common Themes
This section summarizes the expected properties of a DetNet network,
based on the use cases as described in this draft.
11.1. Unified, standards-based network11.1.1. Extensions to Ethernet
A DetNet network is not "a new kind of network" - it based on
extensions to existing Ethernet standards, including elements of IEEE
802.1 AVB/TSN and related standards. Presumably it will be possible
to run DetNet over other underlying transports besides Ethernet, but
Ethernet is explicitly supported.
11.1.2. Centrally Administered
In general a DetNet network is not expected to be "plug and play" -
it is expected that there is some centralized network configuration
and control system. Such a system may be in a single central
location, or it maybe distributed across multiple control entities
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that function together as a unified control system for the network.
However, the ability to "hot swap" components (e.g. due to
malfunction) is similar enough to "plug and play" that this kind of
behavior may be expected in DetNet networks, depending on the
implementation.
11.1.3. Standardized Data Flow Information Models
Data Flow Information Models to be used with DetNet networks are to
be specified by DetNet.
11.1.4. L2 and L3 Integration
A DetNet network is intended to integrate between Layer 2 (bridged)
network(s) (e.g. AVB/TSN LAN) and Layer 3 (routed) network(s) (e.g.
using IP-based protocols). One example of this is "making AVB/TSN-
type deterministic performance available from Layer 3 applications,
e.g. using RTP". Another example is "connecting two AVB/TSN LANs
("islands") together through a standard router".
11.1.5. Guaranteed End-to-End Delivery
Packets sent over DetNet are guaranteed not to be dropped by the
network due to congestion. (Packets may however be dropped for
intended reasons, e.g. per security measures).
11.1.6. Replacement for Multiple Proprietary Deterministic Networks
There are many proprietary non-interoperable deterministic Ethernet-
based networks currently available; DetNet is intended to provide an
open-standards-based alternative to such networks.
11.1.7. Mix of Deterministic and Best-Effort Traffic
DetNet is intended to support coexistance of time-sensitive
operational (OT) traffic and information (IT) traffic on the same
("unified") network.
11.1.8. Unused Reserved BW to be Available to Best Effort Traffic
If bandwidth reservations are made for a stream but the associated
bandwidth is not used at any point in time, that bandwidth is made
available on the network for best-effort traffic. If the owner of
the reserved stream then starts transmitting again, the bandwidth is
no longer available for best-effort traffic, on a moment-to-moment
basis. Note that such "temporarily available" bandwidth is not
available for time-sensitive traffic, which must have its own
reservation.
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Internet-Draft DetNet Use Cases September 201711.1.9. Lower Cost, Multi-Vendor Solutions
The DetNet network specifications are intended to enable an ecosystem
in which multiple vendors can create interoperable products, thus
promoting device diversity and potentially higher numbers of each
device manufactured, promoting cost reduction and cost competition
among vendors. The intent is that DetNet networks should be able to
be created at lower cost and with greater diversity of available
devices than existing proprietary networks.
11.2. Scalable Size
DetNet networks range in size from very small, e.g. inside a single
industrial machine, to very large, for example a Utility Grid network
spanning a whole country, and involving many "hops" over various
kinds of links for example radio repeaters, microwave linkes, fiber
optic links, etc.. However recall that the scope of DetNet is
confined to networks that are centrally administered, and explicitly
excludes unbounded decentralized networks such as the Internet.
11.3. Scalable Timing Parameters and Accuracy11.3.1. Bounded Latency
The DetNet Data Flow Information Model is expected to provide means
to configure the network that include parameters for querying network
path latency, requesting bounded latency for a given stream,
requesting worst case maximum and/or minimum latency for a given path
or stream, and so on. It is an expected case that the network may
not be able to provide a given requested service level, and if so the
network control system should reply that the requested services is
not available (as opposed to accepting the parameter but then not
delivering the desired behavior).
11.3.2. Low Latency
Applications may require "extremely low latency" however depending on
the application these may mean very different latency values; for
example "low latency" across a Utility grid network is on a different
time scale than "low latency" in a motor control loop in a small
machine. The intent is that the mechanisms for specifying desired
latency include wide ranges, and that architecturally there is
nothing to prevent arbirtrarily low latencies from being implemented
in a given network.
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Internet-Draft DetNet Use Cases September 201711.3.3. Symmetrical Path Delays
Some applications would like to specify that the transit delay time
values be equal for both the transmit and return paths.
11.4. High Reliability and Availability
Reliablity is of critical importance to many DetNet applications, in
which consequences of failure can be extraordinarily high in terms of
cost and even human life. DetNet based systems are expected to be
implemented with essentially arbitrarily high availability (for
example 99.9999% up time, or even 12 nines). The intent is that the
DetNet designs should not make any assumptions about the level of
reliability and availability that may be required of a given system,
and should define parameters for communicating these kinds of metrics
within the network.
A strategy used by DetNet for providing such extraordinarily high
levels of reliability is to provide redundant paths that can be
seamlessly switched between, while maintaining the required
performance of that system.
11.5. Security
Security is of critical importance to many DetNet applications. A
DetNet network must be able to be made secure against devices
failures, attackers, misbehaving devices, and so on. In a DetNet
network the data traffic is expected to be be time-sensitive, thus in
addition to arriving with the data content as intended, the data must
also arrive at the expected time. This may present "new" security
challenges to implementers, and must be addressed accordingly. There
are other security implications, including (but not limited to) the
change in attack surface presented by packet replication and
elimination.
11.6. Deterministic Flows
Reserved bandwidth data flows must be isolated from each other and
from best-effort traffic, so that even if the network is saturated
with best-effort (and/or reserved bandwidth) traffic, the configured
flows are not adversely affected.
12. Use Cases Explicitly Out of Scope for DetNet
This section contains use case text that has been determined to be
outside of the scope of the present DetNet work.
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Internet-Draft DetNet Use Cases September 201712.1. DetNet Scope Limitations
The scope of DetNet is deliberately limited to specific use cases
that are consistent with the WG charter, subject to the
interpretation of the WG. At the time the DetNet Use Cases were
solicited and provided by the authors the scope of DetNet was not
clearly defined, and as that clarity has emerged, certain of the use
cases have been determined to be outside the scope of the present
DetNet work. Such text has been moved into this section to clarify
that these use cases will not be supported by the DetNet work.
The text in this section was moved here based on the following
"exclusion" principles. Or, as an alternative to moving all such
text to this section, some draft text has been modified in situ to
reflect these same principles.
The following principles have been established to clarify the scope
of the present DetNet work.
o The scope of network addressed by DetNet is limited to networks
that can be centrally controlled, i.e. an "enterprise" aka
"corporate" network. This explicitly excludes "the open
Internet".
o Maintaining synchronized time across a DetNet network is crucial
to its operation, however DetNet assumes that time is to be
maintained using other means, for example (but not limited to)
Precision Time Protocol ([IEEE1588]). A use case may state the
accuracy and reliability that it expects from the DetNet network
as part of a whole system, however it is understood that such
timing properties are not guaranteed by DetNet itself. It is
currently an open question as to whether DetNet protocols will
include a way for an application to communicate such timing
expectations to the network, and if so whether they would be
expected to materially affect the performance they would receive
from the network as a result.
12.2. Internet-based Applications12.2.1. Use Case Description
There are many applications that communicate across the open Internet
that could benefit from guaranteed delivery and bounded latency. The
following are some representative examples.
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Internet-Draft DetNet Use Cases September 201712.2.1.1. Media Content Delivery
Media content delivery continues to be an important use of the
Internet, yet users often experience poor quality audio and video due
to the delay and jitter inherent in today's Internet.
12.2.1.2. Online Gaming
Online gaming is a significant part of the gaming market, however
latency can degrade the end user experience. For example "First
Person Shooter" (FPS) games are highly delay-sensitive.
12.2.1.3. Virtual Reality
Virtual reality (VR) has many commercial applications including real
estate presentations, remote medical procedures, and so on. Low
latency is critical to interacting with the virtual world because
perceptual delays can cause motion sickness.
12.2.2. Internet-Based Applications Today
Internet service today is by definition "best effort", with no
guarantees on delivery or bandwidth.
12.2.3. Internet-Based Applications Future
We imagine an Internet from which we will be able to play a video
without glitches and play games without lag.
For online gaming, the maximum round-trip delay can be 100ms and
stricter for FPS gaming which can be 10-50ms. Transport delay is the
dominate part with a 5-20ms budget.
For VR, 1-10ms maximum delay is needed and total network budget is
1-5ms if doing remote VR.
Flow identification can be used for gaming and VR, i.e. it can
recognize a critical flow and provide appropriate latency bounds.
12.2.4. Internet-Based Applications Asks
o Unified control and management protocols to handle time-critical
data flow
o Application-aware flow filtering mechanism to recognize the timing
critical flow without doing 5-tuple matching
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o Unified control plane to provide low latency service on Layer-3
without changing the data plane
o OAM system and protocols which can help to provide E2E-delay
sensitive service provisioning
12.3. Pro Audio and Video - Digital Rights Management (DRM)
This section was moved here because this is considered a Link layer
topic, not direct responsibility of DetNet.
Digital Rights Management (DRM) is very important to the audio and
video industries. Any time protected content is introduced into a
network there are DRM concerns that must be maintained (see
[CONTENT_PROTECTION]). Many aspects of DRM are outside the scope of
network technology, however there are cases when a secure link
supporting authentication and encryption is required by content
owners to carry their audio or video content when it is outside their
own secure environment (for example see [DCI]).
As an example, two techniques are Digital Transmission Content
Protection (DTCP) and High-Bandwidth Digital Content Protection
(HDCP). HDCP content is not approved for retransmission within any
other type of DRM, while DTCP may be retransmitted under HDCP.
Therefore if the source of a stream is outside of the network and it
uses HDCP protection it is only allowed to be placed on the network
with that same HDCP protection.
12.4. Pro Audio and Video - Link Aggregation
Note: The term "Link Aggregation" is used here as defined by the text
in the following paragraph, i.e. not following a more common Network
Industry definition. Current WG consensus is that this item won't be
directly supported by the DetNet architecture, for example because it
implies guarantee of in-order delivery of packets which conflicts
with the core goal of achieving the lowest possible latency.
For transmitting streams that require more bandwidth than a single
link in the target network can support, link aggregation is a
technique for combining (aggregating) the bandwidth available on
multiple physical links to create a single logical link of the
required bandwidth. However, if aggregation is to be used, the
network controller (or equivalent) must be able to determine the
maximum latency of any path through the aggregate link.
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Internet-Draft DetNet Use Cases September 201713.5. Cellular Radio
This section was derived from draft-korhonen-detnet-telreq-00.
13.6. Industrial M2M
The authors would like to thank Feng Chen and Marcel Kiessling for
their comments and suggestions.
13.7. Internet Applications and CoMP
This section was derived from draft-zha-detnet-use-case-00.
This document has benefited from reviews, suggestions, comments and
proposed text provided by the following members, listed in
alphabetical order: Jing Huang, Junru Lin, Lehong Niu and Oilver
Huang.
13.8. Electrical Utilities
The wind power generation use case has been extracted from the study
of Wind Farms conducted within the 5GPPP Virtuwind Project. The
project is funded by the European Union's Horizon 2020 research and
innovation programme under grant agreement No 671648 (VirtuWind).
13.9. Network Slicing
This section was written by Xuesong Geng, who would like to
acknowledge Norm Finn and Mach Chen for their useful comments.
13.10. Mining
This section was written by Diego Dujovne in conjunction with Xavier
Vilasojana.
13.11. Private Blockchain
This section was written by Daniel Huang.
14. Informative References
[ACE] IETF, "Authentication and Authorization for Constrained
Environments",
<https://datatracker.ietf.org/doc/charter-ietf-ace/>.
[Ahm14] Ahmed, M. and R. Kim, "Communication network architectures
for smart-wind power farms.", Energies, p. 3900-3921. ,
June 2014.
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